JP2983535B1 - Optical scanning device - Google Patents

Abstract

Kind Code: A1 The first and second scanning lenses are injection-molded from plastic, and the curvature of field on the surface to be scanned is satisfactorily corrected. Provided is an optical scanning device which has a relatively simple lens shape and is easy to mold and injection mold while minimizing. SOLUTION: A first surface 16a of a first scanning lens 16 is formed as a rotationally symmetric aspherical surface. First scanning lens 16
The second surface 16b and the third surface 17a and the fourth surface 17b of the second scanning lens 17 are formed as aspheric toric surfaces having a rotation axis in the main scanning direction. The shapes of the scanning lenses 16 and 17 near the optical axis are concave, convex, convex, and concave, respectively, from the first surface 16a to the fourth surface 17b in the main scanning direction, and are formed in the sub-scanning direction. The first surface 16a to the fourth surface 17b are configured as concave portions, convex portions, concave portions, and convex portions, respectively.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to an optical scanning device. More specifically, it has a scanning optical system composed of two plastic lenses, the curvature of field on the surface to be scanned is corrected well, and the focus movement due to a temperature change, the eccentricity, and the deterioration of the optical performance due to a tilt error are reduced. The present invention relates to an optical scanning device that is minimized, has a relatively simple lens shape, and is easy to mold and injection mold.

[0002]

2. Description of the Related Art In a conventional optical scanning device, as shown in FIG. 1, a light beam modulated by a laser diode 1 is converted into a parallel beam while passing through a collimator lens 2. Thereafter, the light passes through a long-slit slit 3 in the main scanning direction of the plane. The parallel luminous flux having passed through the slit 3 is incident on a cylinder lens 4 having a refracting power in the sub-main scanning direction on the front. After passing through the cylinder lens 4, the light beam in the main scanning direction enters the reflecting surface of the rotating polygon mirror 5 as parallel light and is deflected by the scanning lens system 6, while the light beam in the sub-scanning direction is reflected by the rotating polygon mirror 5. After being imaged on the surface, it is deflected by the scanning lens system 6.

[0003] The role of the cylinder lens 4 is that when each reflecting surface of the rotary polygon mirror 5 is deviated at an angle inclined with respect to the rotation axis, a light beam is formed on the surface to be scanned in the sub-scanning direction. Another object of the present invention is to minimize the amount of deviation in beam diameter. In Japanese Patent Application Laid-Open No. 4-110817, a toric surface having different radii of curvature in the main scanning direction and the sub-scanning direction is used for the scanning lens system in order to minimize the effect of blurring, and the toric surface is rotated in the sub-scanning direction. It has been proposed that an optical conjugate relationship be established between the reflection surface of the polygon mirror and the scanned surface 7.

However, Japanese Patent Application Laid-Open No. 4-110817 has good performance such as spot size and linearity, but the entire lens surface is formed as a toric surface. Performance degradation is expected.

Also, US Pat. No. 4,639,072 discloses that
A technique is disclosed in which a cylinder lens is arranged near the surface to be scanned 7 to prevent adverse effects on performance due to blurring.
The scanning lens system forms an elliptical spot having a long axis in the sub-scanning direction on the surface to be scanned with the light beam deflected by the rotating polygon mirror, and has f · θ characteristics, that is,

(Equation 1) Meet.

However, when the rotating polygon mirror rotates, the position of the deflecting surface changes, the above-mentioned optical conjugate relationship is not maintained, and the image forming point in the sub-scanning direction is shown asymmetrically on the surface to be scanned. Occurs. further,
In U.S. Pat. No. 5,488,502, an image of a surface to be scanned in the sub-scanning direction is formed by using a deformed cylinder lens in which the radius of curvature in the sub-scanning direction is asymmetrical as a scanning lens near the surface to be scanned. There is a proposal to minimize positional deviation at points. According to the publication, the distance from the deflecting surface to the surface to be scanned is shorter than that of the conventional optical system, but the distance between the lenses is increased in order to improve the characteristics and the curvature of field. Becomes difficult.

Generally, the ratio of the focal length (f) to the distance L from the deflection point to the image plane is L / f <, while maintaining high optical performance, wide angle and good f · θ characteristics. In the case of about 1.33, the size of the unit can be reduced. However, as the distance from the center to both ends of the surface to be scanned increases, it becomes more difficult to correct the difference in reception and the spot size is increased. Performance degradation occurs at both ends of the surface.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to form the first and second scanning lenses by injection molding of plastic and improve the curvature of field on the surface to be scanned. It is corrected, while minimizing the focus movement and eccentricity due to temperature change, and the decrease in optical performance due to tilt error,
An object of the present invention is to provide an optical scanning device having a relatively simple lens shape and easy mold processing and injection molding.

[0009]

In order to achieve the above object, the present invention provides a light source for scanning light-modulated light from a laser diode, and a collimator lens for converting the scanned light from the light source into parallel light. A slit that passes only a necessary portion of the parallel light passing through the collimator lens during scanning, a cylinder lens that converges the parallel light that has passed through the slit in the sub-scanning direction, and a main lens that passes through the cylinder lens. A rotating polygon mirror which is parallel to the scanning direction and is rotated at high speed so that when light converged in the sub-scanning direction is incident, the light is deflected at a certain angle; and And a scanning lens system for converging the light beam in the main scanning and sub-scanning directions and forming an image on the surface to be scanned when the light beam deflected by the scanning lens system is incident. It consists of two sheets of the first scanning lens and the second scanning lens sticks material, on a first face of the first scanning lens is rotationally symmetrical aspheric or spherical, first
Second surface of scanning lens, third surface and fourth surface of second scanning lens
The gist of the invention is that the surface is constituted by an aspheric toric surface having a rotation axis in the main scanning direction.

[0010]

FIG. 2 is a perspective view of an optical scanning device according to a first embodiment of the present invention, and FIG. 3 is a plan view and a front view of the optical scanning device according to the first embodiment of the present invention. Light source 1
The scanned beam from 0 is the collimating lens 1
1, through a slit 12 and a cylindrical lens 13, the light is incident on a rotary polygon mirror 14 in a state parallel to the main scanning direction and converged in the sub-scanning direction. The rotary polygon mirror 14 deflects the incident light beam at a constant angular velocity. The deflected light beam is converged thereafter while being scanned by the scanning lens system 15 and forms a focal point on the surface 18 to be scanned. An optical conjugate relationship is set in the sub-scanning direction between the deflection point of the rotary polygon mirror 14 and the scan surface 18, and the reflection surface of the rotary polygon mirror 14 is rotated by the rotation axis of the rotary polygon mirror 14. , The focal point shift in the sub-scanning direction on the scanned surface 18 is minimized.

Generally, the first scanning lens 16 and the second scanning lens 17 constituting the scanning lens system 15 are each formed of a plastic lens. However, the plastic lens is inexpensive, but has a low humidity. The weaker the refractive power, the greater the decrease in optical performance due to thermal deformation,
There is a disadvantage. In order to overcome this, the present invention uses zeonics or Aton plastics having improved thermal characteristics, humidity, birefringence, internal distortion and the like. And
The shape of the first scanning lens 16 and the second scanning lens 17 is
It is configured as follows.

The surface configuration of the scanning lens system 15 is as follows. A first surface 16a of the first scanning lens 16, that is, a surface facing the rotary polygon mirror 14 is formed of a rotationally symmetric aspherical or spherical surface, and a second surface 16b is an aspherical surface having a rotation axis in the main scanning direction. It consists of a toric surface. Also, the second
The third surface 17a and the fourth surface 17b of the scanning lens 17 are each formed of an aspheric toric surface having a rotation axis in the main scanning direction. Further, the shape of the scanning lens system in the vicinity of the optical axis includes a concave portion, a convex portion, a convex portion, and a concave portion from the first surface to the fourth surface in the main scanning direction, and the first surface to the fourth surface in the sub-scanning direction. It is characterized in that each of the four surfaces is constituted by a concave portion, a convex portion, a concave portion and a convex portion. Here, the shape formula of the aspheric toric surface is as follows.

(Equation 2)

(Equation 3)

(Equation 4) Equation (1) is a shape equation of the first surface 16a of the scanning lens system, and equations (2) and (3) are equations of the second surface 16a of the scanning lens system.
b, the third surface 17a, and the fourth surface 17b.

Here, x and y are arbitrary points on the X and Y axes (see FIG. 2), and z in equation (1) is an optical axis direction from an arbitrary point on the rotationally symmetric aspheric surface to the XY plane. Distance (SAG amount)
Where z in equation (3) indicates the distance (SAG amount) in the optical axis direction from any point on the aspheric toric to the XY plane, and Zy is the distance from any point on the aspheric toric to the Y axis. The distance in the optical axis direction (see FIG. 4), R is the radius of curvature of the main scanning near the optical axis, R 'is the radius of curvature of the sub-scanning, k is the conic surface coefficient, A is the fourth order aspherical coefficient, and B is 6 The next aspheric coefficient, C
Denotes an eighth-order aspheric coefficient. Here, K = A = B = C =
If 0, Equation 1 represents a spherical surface.

According to the present invention, when the focal lengths of the first scanning lens and the second scanning lens in the main scanning direction in the scanning optical system are f1 and f2, respectively, 0.3 <| f1 / f2 | <0.5 Satisfies the condition.

In the scanning optical system, the focal lengths of the first scanning lens and the second scanning lens in the sub-scanning direction are each set to f1.
, F2 ', the condition 1.0 <| f1' / f2 '| <2.0 is satisfied.

In the scanning optical system, the focal length of the first scanning lens in the main scanning direction is f1, and the focal length in the sub-scanning direction is f.
When 1 ′, the condition of 1.8 <| f1 / f1 ′ | <2.8 is satisfied.

In the scanning optical system, the focal length of the second scanning lens in the main scanning direction is f2, and the focal length in the sub-scanning direction is f.
2 ′, the condition of 7.5 <| f2 / f2 ′ | <10.5 is satisfied.

The scanning lens system configured as described above includes:
The f · θ characteristic and the curvature of field of the scanned surface are favorably corrected at a wide angle, and the deterioration of the optical performance on the scanned surface due to eccentricity and tilt error between the scanning lenses or the scanning lenses is minimized. At this time, since the aspherical toric surface is a surface whose rotation axis is the main scanning direction, it is easy to mold a precise shape, and since the surface shape is relatively simple, injection molding is performed. Is easy. In addition, a proper refractive power distribution (reciprocal of the focal length) between the scanning lenses can minimize the focus movement due to a temperature change.

The design data of the first embodiment of the present invention is as follows.

[Table 1] In the first embodiment of the present invention, the operating wavelength of the light source is 78
6.5 nm, and the angle of incidence of the rotating polygon mirror is 80.0 °
(See FIG. 2). The maximum scanning angle (θ) of the rotary polygon mirror is ± 45 °, the focal length (f) of the scanning lens is 136 mm, and the distance from the deflection point (p) of the rotary polygon mirror to the surface to be scanned. Is 185.0 mm (see FIG. 3b). The rotating polygon mirror has six surfaces, the diameter of the inscribed circle is φ34.64 mm, and the size of the slit is 2.6 (main scanning direction) × 1.5 (sub scanning direction).

When the focal lengths of the first scanning lens and the second scanning lens in the main scanning direction are f1 and f2, respectively, f1 / f2 = 0.44, and the first scanning lens and the second scanning lens have the same focal length. The focal length in the sub-scanning direction is f1
Where f1 ′ / f2 ′ = 1.39, f1 / f1 ′ = 2.52, and f2 / f2 ′
= 7.94.

The curvature of field and the f · θ characteristics shown by the first embodiment of the present invention, that is,

(Equation 5) Are as shown in FIG. 5 and FIG.

FIGS. 7A and 7B are a plan view and a front view, respectively, of the optical scanning device according to the second embodiment of the present invention. The design data is as follows.

[Table 2] In the second embodiment of the present invention, the operating wavelength of the light source is 78
6.5 nm, and the angle of incidence of the rotating polygon mirror is 80.0 °
(See FIG. 2). The maximum scanning angle (θ) of the rotary polygon mirror is ± 45 °, the focal length (f) of the scanning lens is 136.0 mm, and the distance from the deflection point (p) of the rotary polygon mirror to the surface to be scanned. Is 185.0 mm (see FIG. 3b). The rotating polygon mirror has six surfaces, the diameter of the inscribed circle is φ34.64 mm, and the size of the slit is 2.6 (main scanning direction) × 1.5 (sub scanning direction).

When the focal lengths of the first scanning lens and the second scanning lens in the main scanning direction are f1 and f2, respectively, f1 / f2 = 0.35, and the first scanning lens and the second scanning lens have the same focal length. The focal length in the sub-scanning direction is f1
Where f1 ′ / f2 ′ = 1.71, f1 / f1 ′ = 2.03, and f2 / f2 ′
= 10.04.

The field curvature and the f · θ characteristic shown by the second embodiment of the present invention, that is,

(Equation 6) Are as shown in FIG. 8 and FIG.

According to the first and second embodiments of the present invention, the first surface 16 of the first scanning lens 16 pointing toward the rotary polygon mirror 14 is used.
a is a shape depressed in the main scanning and sub-scanning directions.
The surface 16b has a shape bulging in the main scanning and sub-scanning directions. The third surface 17a of the second scanning lens 17 pointing toward the surface to be scanned 18 has a shape bulging in the main scanning direction and a shape concave in the sub-scanning direction, and the fourth surface 17b has a shape corresponding to the main scanning direction. The shape is concave in the direction and swells in the sub-scanning direction.

[0026]

As described in detail above, when the scanning lens system of the present invention is applied to an image forming apparatus such as a laser beam printer, environmental changes, manufacturing errors during lens manufacturing, assembly errors in units, etc. , The deterioration of the optical performance due to the above can be minimized, the mold and the injection molding are relatively easy, and it is possible to mass produce low-cost lenses.

[Brief description of the drawings]

1A and 1B are plan views showing a conventional optical scanning device.

FIG. 2 is a perspective view illustrating the optical scanning device according to the first embodiment of the present invention.

FIGS. 3A and 3B are a plan view and a front view illustrating the optical scanning device according to the first embodiment. FIGS.

FIG. 4 is a diagram showing a distance and a focal length of a scanning lens in an optical axis direction.

FIG. 5 is a diagram illustrating curvature of field in main scanning and sub-scanning according to the first embodiment.

FIG. 6 is a diagram illustrating f · θ characteristics of the scanning lens according to the first embodiment.

FIGS. 7A and 7B are a plan view and a front view illustrating an optical scanning device according to a second embodiment of the present invention.

FIG. 8 is a diagram illustrating curvature of field in main scanning and sub-scanning according to the second embodiment.

FIG. 9 is a diagram showing f · θ characteristics of the scanning lens according to the second embodiment.

[Explanation of symbols]

Reference Signs List 10 light source, 11 collimating lens, 12 slit, 13 cylindrical lens, 14 rotating polygon mirror, 15 scanning lens system, 16 first scanning lens, 16
a: first surface, 16b: second surface, 17: second scanning lens,
17a: third surface, 17b: fourth surface, 18: surface to be scanned.

Claims (10)

(57) [Claims] 1. A light source that scans light modulated from a laser diode, a collimator lens that converts a light beam scanned from the light source into parallel light, and a light source that is required for scanning among parallel light passing through the collimator lens. A slit that passes only a part of the light, a cylinder lens that converges parallel light that has passed through the slit in the sub-scanning direction, and light that is parallel to the main scanning direction and converges in the sub-scanning direction while passing through the cylinder lens. When a light is deflected by the rotating polygon mirror, the rotating polygon mirror is rotated at a high speed so that the light is deflected at a certain angle. A scanning lens system that converges in the scanning and sub-scanning directions and forms an image on a surface to be scanned, wherein the scanning lens system is a first scanning lens and a second scanning lens made of a plastic material And the first surface of the first scanning lens is a rotationally symmetric aspherical surface, while the second surface of the first scanning lens and the third and fourth surfaces of the second scanning lens are mainly An optical scanning device comprising an aspheric toric surface having a rotation axis in a scanning direction. 2. The shape of the scanning lens in the vicinity of the optical axis includes a concave portion, a convex portion, and a first surface to a fourth surface in the main scanning direction.
The optical scanning device according to claim 1, wherein the optical scanning device includes a convex portion and a concave portion. 3. The shape of the scanning lens in the vicinity of the optical axis includes a concave portion, a convex portion, and a first surface to a fourth surface, respectively, in the sub-scanning direction.
The optical scanning device according to claim 1, wherein the optical scanning device includes a concave portion and a convex portion. 4. A light source for scanning light modulated light from a laser diode, a collimator lens for converting a light beam scanned from the light source into parallel light, and a light source for scanning among parallel light passing through the collimator lens. A slit that passes only a part of the light, a cylinder lens that converges parallel light that has passed through the slit in the sub-scanning direction, and light that is parallel to the main scanning direction and converges in the sub-scanning direction while passing through the cylinder lens. When a light is deflected by the rotating polygon mirror, the rotating polygon mirror is rotated at a high speed so that the light is deflected at a certain angle. A scanning lens system that converges in the scanning and sub-scanning directions and forms an image on a surface to be scanned, wherein the scanning lens system is a first scanning lens and a second scanning lens made of a plastic material And the first surface of the first scanning lens is spherical, while the second surface of the first scanning lens and the third and fourth surfaces of the second scanning lens rotate in the main scanning direction. An optical scanning device comprising an aspheric toric surface having an axis. 5. The shape of the scanning lens near the optical axis includes a concave portion, a convex portion, and a first surface to a fourth surface, respectively, in the main scanning direction.
The optical scanning device according to claim 4, wherein the optical scanning device includes a convex portion and a concave portion. 6. The shape of the scanning lens in the vicinity of the optical axis includes a concave portion, a convex portion, and a first surface to a fourth surface, respectively, in the sub-scanning direction.
The optical scanning device according to claim 4, wherein the optical scanning device includes a concave portion and a convex portion. 7. The scanning lens system, wherein the focal lengths of the first scanning lens and the second scanning lens in the main scanning direction are respectively f
5. The condition of 0.3 <| f1 / f2 | <0.5 is satisfied when 1 and f2 are satisfied. 5.
3. The optical scanning device according to claim 1. 8. The focal length of the first scanning lens and the second scanning lens in the sub-scanning direction in the scanning lens system is set to f.
5. The condition of 1.0 <| f1 ′ / f2 ′ | <2.0 when 1 ′ and f2 ′ is satisfied. 5.
3. The optical scanning device according to claim 1. 9. When the focal length of the first scanning lens in the scanning lens system in the main scanning direction is f1 and the focal length in the sub-scanning direction is f1 ′, 1.8 <| f1 / f1 ′ | <2. 6. The condition of claim 1 or claim 4, wherein the following condition is satisfied.
3. The optical scanning device according to claim 1. 10. When the focal length in the main scanning direction of the second scanning lens in the scanning lens system is f2 and the focal length in the sub-scanning direction is f2 ′, 7.5 <| f2 / f2 ′ | <10. 5. The method according to claim 1 or 4, wherein the following condition is satisfied.
3. The optical scanning device according to claim 1.